Excel switching day and month around after entry.

I have a spreadsheet where the column formats are determined by vba code.

Sheets(1).Columns(1).NumberFormat = "ddd dd/mm/yy"
Sheets(1).Columns(8).NumberFormat = "dd/mm/yyyy"

Recently I have noticed if I type 3/5 for 3rd of May it switches this around and displays 05/03/2020.

I can't figure out how to stop this. Everything is formatted for UK date and locale, but excel think I'm entering the date American style and "corrects" it for me.

I tried a new spreadsheet and set column format to dd/mm/yyyy and it doesn't switch the date around after entry.

Can anyone help me fix this?

Does anyone have any suggestions to help with this? Despite searching I cannot find a solution.